Mercury cadmium telluride infrared filters and detectors and methods of fabrication

ABSTRACT

A multiple film integrated infrared (IR) detector assembly 85 consists of detector films 86, 88, 90 having different IR spectral sensitivities which are deposited on a breadboard IR transmissive but electrically insulating substrate 42. Substrate 42 is deposited on an IR filter layer comprising an HgCdTe film 70. By various techniques described, filter film 70 has a varying composition from edge 68 to 72. This compositional gradient of film 70 results in varying IR spectral absorption as shown by IR transmission graphs 10, 12, 14. Film 70 acts as a graded IR filter in concert with the response of the detector films 86, 88, 90. By the proper choice of the compositional gradients in these films, and as a result the IR spectral response, an integrated IR spectrometer may be fabricated whereby each detector 86, 87, 90 detects only specific narrow bands of IR wavelengths.

This application claims priority under 35 USC 119 (e) (1) of provisionalapplication number 60/014,873 filed Apr. 4, 1996.

Cross-reference to Commonly-owned, Co-filed, Related Applications

The following are commonly-owned, co-filed, related applications, andare incorporated by reference herein; "NARROW BAND INFRARED FILTERDETECTORS" U.S. Ser. No. 08/831,103 filed Apr. 1, 1997; "MERCURY CADMIUMTELLURIDE DEVICES FOR DETECTING AND CONTROLLING OPEN FLAMES" U.S. Ser.No. 08/834,791, filed Apr. 3, 1997; "UNCOOLED MERCURY CADMIUM TELLURIDEINFRARED DEVICES WITH INTEGRAL OPTICAL ELEMENTS" U.S. Ser. No.08/834,790, filed Apr. 3, 1997; "UNCOOLED INFRARED SENSORS FOR THEDETECTION AND IDENTIFICATION OF CHEMICAL PRODUCTS OF COMBUSTION" U.S.Ser. No. 08/831,101, filed Apr. 1, 1997; "A METHOD OF FABRICATING ALATERALLY CONTINUOUSLY GRADED HgCdTe LAYER" U.S. Ser. No. 08/831,813,filed Apr. 2, 1997; "INTEGRATED IR DETECTOR SYSTEM" U.S. Ser. No.08/831,815, filed Apr. 2, 1997; and "NARROW BAND INFRAREDFILTER-DETECTORS" U.S. Ser. No. 08/831,814, filed Apr. 2, 1997(abandoned).

FIELD OF THE INVENTION

This invention generally relates to devices for the detection ofinfrared (IR) radiation (e.g. of flames to signal hazardous conditions,or to control of manufacturing processes or to perform spectralanalysis) and, more specifically, to the design and fabrication of suchdevices.

BACKGROUND OF THE INVENTION

When materials burn or explode, emission of light in the visible as wellas the invisible infrared (IR) and ultraviolet (UV) wavelengths occur.The chemical composition of the burning flame determines the wavelengthsof light emitted. These emissions can be detected by variousphotosensitive devices for safety, process control or spectroscopicpurposes.

Fire detection systems which furnish an electrical output signal inresponse to a sudden flame or explosion are well known. Such systems areavailable on the open market, but are high cost items. One of thereasons for this high cost has been the low sensitivity from availabledetectors as well as the high cost of detector manufacture. The lowsensitivity results in low signal to noise ratio of the system whichcauses a high rate of false alarms. To circumvent the problem of falsealarms, the use of individual detectors having different spectralresponses has been taught by Kern, et al (see U.S. Patents to Kern etal.: Pat. No. 4,296,324 entitled "DUAL SPECTRUM INFRARED FIRE SENSOR",issued Oct. 20, 1981; Pat. No. 4,691,196 entitled "DUAL SPECTRUMFREQUENCY RESPONDING FIRE SENSOR", issued Sep. 1, 1987; Pat. No.4,769,775 entitled "MICROPROCESSOR-CONTROLLED FIRE SENSOR", issued Sep.6, 1988; and Pat. No. 4,785,292 entitled "DUAL SPECTRUM FREQUENCYRESPONDING FIRE SENSOR", issued Nov. 15, 1988). In addition, intensitycomparisons have been made between UV and IR wavelengths to furtherreduce false alarms. Complex microprocessor logic has been employed toanalyze the flicker frequency of the radiation to distinguish a flamefrom background IR emission.

Axmark, et al (see U.S. Pat. to Axmark et al. Pat. No. 4,370,557entitled "DUAL DETECTOR FLAME SENSOR" issued Jan. 25, 1983) teaches asystem using dual, individual, dissimilar detectors for the control of amulti-burner boiler or industrial furnace installation. The detectorsused in Axmark were a silicon (Si) detector responsive to visible lightand an IR responsive lead-sulfide (PbS) detector with emphasis on theuse of both the direct current (dc) and alternating current (ac)responses of these detectors.

In medical research and chemical analysis, IR spectroscopy is oftenuseful. Instruments to perform this type of analysis typically cost$10,000.00 in 1994 US dollars.

Military applications are another expensive use of IR detection systems.Such systems are generally used for IR imaging similar to radar or forthe guidance of heat seeking missiles. Although many different materialsare used for these detectors, one of these is mercury-cadmium-telluride,HgCdTe, hereafter referred to as MCT. MCT detectors are cooled wellbelow atmospheric temperatures, typically 77 Kelvin, to accomplishdetectivity of targets near atmospheric temperature.

SUMMARY OF THE INVENTION

The present invention relates to the growth, preparation and applicationof thin films of MCT as IR filters and detectors in an uncooledenvironment for detection, control and analysis.

One of the physical attributes that makes MCT so favorable as an IRfilter and/or detector is that a thin film of MCT exhibits a significantchange of electrical conductivity when exposed to certain wavelengths ofIR radiation. Another important attribute of MCT is that the IRtransmission and electrical conductance properties versus IR wavelengthmay be controlled by the ratio of Hg to Cd in the MCT film. At shorterIR wavelengths than chosen for the fabricated film composition, IRradiation will be greatly attenuated at the same time the electricalconductance will be significantly increased. With little attenuation,the MCT film transmits IR radiation at longer wavelengths than thewavelengths which cause the film conductivity to change. These usefulphysical attributes have been applied to several embodiments described,e.g., in the aforementioned co-filed applications, (U.S. Ser. Nos.08/834,791 and 08/831,101). Many of those previous embodiments utilizecontrolling the composition of MCT during film deposition to obtain thedesired IR spectral selectivity. This was often achieved by a controlledchange of MCT composition through the thickness of the MCT film. Whileseveral methods of epitaxial film growth exists, it is known that liquidphase epitaxial growth (LPE) in a Hg rich environment provides superiorfilms to other methods, (refer to T. Ting, Journal of Crystal Growth,(Netherlands), vol. 86, pp 161-172, 1988!.) This invention describesseveral novel embodiments for achieving a controlled compositionalgradient through the thickness of the film during and after film growth.

The use of accurately controlled diamond point turning to lap MCTmaterial at a shallow angle with respect to the surface to produce awedge-like lateral structure translates the vertical compositionalgradient into the lateral dimension. Embodiments of IR optical filterand detector structures will be disclosed which use this technique bothwith single and multiple films of MCT.

Other embodiments show how this lateral compositional gradient may beachieved more directly by means of post deposition heat treatment in anenvironment with a controlled lateral temperature gradient. This lattermethod also provides a family of novel IR filters having sharply tunedwavelength responses which are not achievable by other methods.

For composite active elements, such as silicon (Si) integrated circuitscombined with MCT IR filters and/or detectors, the lattice mismatchbetween Si and MCT is so large that epitaxial growth of either on theother is impossible. Methods are shown whereby suitable intermediarylayers provide a novel material structure that allows the combination ofactive elements using either or both gallium arsenide (GaAs) and Si withMCT films.

Some embodiments include an integrated IR detector assembly, with theassembly comprising: (a) a laterally continuously graded HgCdTe filterlayer; (b) an electrically insulating layer directly on the continuouslygraded HgCdTe film; and (c) at least two integral HgCdTe detectorsdirectly on the electrically insulating layer.

The electrically insulating layer (b) is preferably CdTe or CdZnTe. Thehorizontally, continuously graded HgCdTe filter layer (a) is preferablywedge-shaped with the composition continuously graded vertically throughthe thickness of the film.

In some embodiments, the end portions of the detectors preferably haveHgTe ohmic contacts thereon.

In some embodiments it is preferred that the laterally continuouslygraded HgCdTe filter layer be deposited on an IR transmissive window.This IR transmissive window is preferably CdTe.

In some embodiments, a novel method of fabrication is preferred tofabricate an IR detector assembly by the utilization of sequentialliquid epitaxial growth to produce a vertically continuously gradedcomposition HgCdTe filter layer, an electrically insulating layer,preferably CdTe or CdZnTe, and a HgCdTe detector film. Such multiplefilm layers are preferably followed by etching the HgCdTe detector filmlayer to form at least two HgCdTe detectors.

In some embodiments, the growth of this multiple film structure ispreferably followed by diamond point turning the vertically continuouslygraded HgCdTe filter layer to form a laterally continuously gradedHgCdTe filter layer. The laterally continuously graded HgCdTe filterlayer is preferably on an IR transmissive window. The IR transmissivewindow is preferably CdTe.

In some preferred methods of fabrication, the liquid epitaxially growingof the vertically continuously graded HgCdTe filter layer is grownfirst, the electrically insulating layer is grown next, and the HgCdTedetector film is grown last.

In another preferred means of fabrication the liquid epitaxially growingof the HgCdTe detector film is grown first, the electrically insulatinglayer is grown next, and the vertically continuously graded HgCdTefilter layer is grown last.

In another preferred method of forming IR detectors, the etching of theHgCdTe detector film to form at least two HgCdTe is performed prior tothe diamond point turning of the vertically continuously graded HgCdTefilter layer.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, which form an integral part of the specification andare to be read in conjunction therewith, and in which like numerals andsymbols are employed to designate similar components in various viewsunless otherwise indicated:

FIG. 1 shows a graph of IR film properties versus MCT film composition(prior art.)

FIG. 2 is a sketch of a liquid phase epitaxial reactor suited to thegrowth of MCT films in a Hg rich environment (prior art.)

FIG. 3 is an illustration of growth temperature versus time during MCTfilm deposition to create a controlled composition versus thickness duethe sharp dependence of the segregation coefficient of Cd versustemperature.

FIG. 4 is a sketch of post deposition treatment of MCT films depositedon Cd bearing substrates whereby the Cd is driven from the substrate tothe MCT film by interdiffusion.

FIG. 5 shows a variant of the technique shown in FIG. 4 whereby verylong lateral compositional gradients may be achieved in MCT films.

FIG. 6A is a sketch of a continuous MCT film having a compositionalgradient in the film thickness direction (produced by any means) whichhas been lapped with a diamond point turning machine to convert the MCTfilm to an IR filter which has different wavelength responses in thelateral direction;

FIG. 6B sketches the formation of MCT detectors over the filter of FIG.6A by means of conventional photolithography after MCT film deposition;

FIG. 6C sketches the formation of MCT detectors over the filter of FIG.6A through the use of a silicon dioxide mask which is applied andpatterned prior to film deposition to inhibit MCT film growth;

FIG. 7A is a sketch of a substrate holder which allows the simultaneousdeposition of MCT films of the same composition on both sides of one ormore substrates;

FIG. 7B illustrates the post deposition heat treatment of the doublefilm of FIG. 7A to achieve a very long lateral IR filter having novelresponse characteristics;

FIG. 8 is a sketch illustrating the patterning of one of the heattreated double MCT films of FIG. 7A into discrete IR detectors which aredisplaced laterally at the chosen wavelengths with a width selected forthe desired bandwidth;

FIG. 9A illustrates the use of GaAs as a lattice match material betweenMCT and Si to provide a material with active elements to process thesignals from MCT detectors;

FIG. 9B illustrates the use of zinc selenide (ZnSe) as a lattice matchmaterial between MCT and Si to provide a material with active elementsto process the signals from MCT detectors;

FIG. 9C illustrates the use of a latterly continuously graded HgCdTefilter over MCT detectors; and FIGS. 9D-E illustrate rapid thermalannealing lamps used in conjunction with a reflective mask to providenon-uniform heating.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

This description of the preferred embodiments of the present inventionis aided by referral to the accompanying drawings, and Tables 1-5. Inthe sketches showing MCT films, the thickness and wedge tapers aregreatly exaggerated and the scales are neither absolute nor relative.

FIG. 1 is a graph of a model of the IR transmission (%) versuswavenumber (cm⁻¹) for the MCT film composition (Hg.sub.(1-x) Cd_(x))Te.The IR wavelength in mm is -10⁴ divided by the wavenumber. For graph 10,x=0.3. For graph 12, x=0.25. For graph 14, x=0.2. The abscissa of thegraph runs from a wavenumber of -3500 (corresponding to a wavelength of2.9 mm) to a wavenumber of -500 (corresponding to a wavelength of 20mm.) The transmission percentages modeled are illustrative but less thanactual because no anti-reflection coatings are assumed. The wavelengths15 to the left of 10 (4.0 mm and shorter) are absorbed by MCT film 10and, if electrodes are attached, an increase in conductance of 10 may bedetected when emission in the 15 spectral region illuminates the MCTfilm. Conversely, wavelengths in regions 16, 17 and 18 which are longerthan 4.0 mm may be transmitted through film 10 and typically, no changeof conductance in 10 occurs when illuminated at wavelengths longer than4.0 mm within spectral regions 16, 17 and 18. The novel customization ofMCT IR detectors for specific wavelengths to be described for thepresent invention makes use of these physical properties.

FIG. 2 illustrates the preferred and well known method of epitaxiallygrowing superior MCT films having very low Hg vacancies using a liquidphase epitaxial (LPE) reactor. Table 1 describes the key components andtheir function.

The present invention is shown in FIGS. 3-9e. A first embodiment is amethod of varying the composition of a (Hg.sub.(1-x) Cd_(x) Te filmduring LPE film growth makes use of the strong temperature dependence ofthe segregation coefficient of Cd in the MCT melt. FIG. 3 illustrates anexample of a temperature profile to perform this within the reactorillustrated in FIG. 2. During zone 30 the temperature may be held at T1,for example, 450° C., for a rolled period of time, for example 20minutes, to time 36. At time 36, the temperature is linearly

                  TABLE 1    ______________________________________    ID#  Description      Function    ______________________________________    20   Process chamber  Seals process environment    21   Substrate and holder support                          Supports and rotates substrate for                          uniform film deposition.    22   Substrate holder Supports substrate during                          deposition    23   CdTe or CdZnTe substrate                          Provides surface for epitaxially                          growing MCT.    24   Stirrer          Agitates HgCdTe melt 27 for                          controlled deposition on                          substrate 23    25   Hg, Cd, Te vapor Provides Hg vapor rich environ-                          ment during growth of MCT film.    26   Crucible         Container for melt 27    27   Melt             Molten HgCdTe source for MCT                          film deposition.    28   Chamber Heater   Controls temperature of upper                          process chamber.    29   Thermocouple well                          Monitors temperature of chamber                          heater 28.    ______________________________________

reduced in zone 32 until time 38, for example 35 minutes from time 36 totime 38. At time 38 the temperature is once more held constant at T2,for example 400° C., during zone 34 which continues to the thickness ofthe film desired. As the temperature is reduced, the proportion of Cdversus Hg in the MCT film increases because of the differences insegregation coefficients in these two components. With the temperatureprofile sketched in FIG. 3 the value of x in (Hg.sub.(1-x) Cd_(x)) Te issmaller in zone 30 than 34. During zone 32 the content of Cd in the(Hg.sub.(1-x) Cd_(x))Te film linearly increases. This provides the meansof controlling the optical and conductance properties shown in FIG. 1through the thickness of the MCT film during growth. The MCT film nextto the substrate can be either Cd enriched or Cd depleted depending onthe temperature profile during film growth.

A second embodiment of the present invention is a method of providing anMCT film compositional gradient through the film thickness is performedafter film growth. This method makes use of the interdiffusion of Cdfrom the substrate into the MCT film. In FIG. 4 is sketched a diagram ofthe apparatus. An MCT film 40 has been grown on substrate 42. The IRtransparent substrate 42 may be CdTe or cadmium-zinc-telluride (CdZnTe,for example.) The substrate 42 is placed in contact with heater 43 whichis at a raised temperature. This causes the Cd in the substrate 42 tointerdiffuse into MCT film 40 such that the MCT composition next to thesubstrate contains more Cd than the surface of 40. The x in(Hg.sub.(1-x) Cd_(x))Te next to the substrate is larger so thewavelength of IR absorbed or detected at the substrate interface surfaceis shorter than at the top surface of the film. This is the desirablematerial profile for through the substrate 42 IR radiation detectionbecause the longer IR wavelengths must pass through the material next to42 to reach the material at the film surface which is sensitive tolonger wavelengths.

A third embodiment of the present invention is illustrated in FIG. 5 andTable 2, which

                  TABLE 2    ______________________________________    ID#  Description       Function    ______________________________________    39   Zone of 40 having highest Cd                           Provides shortest IR wavelength                           response    40   Uniformly deposited MCT film                           Controls range of IR response                           desired    41   Zone of 40 having lowest Cd                           Provides longest IR wavelength                           response    42   CdTe or CdZnTe substrate                           Provides a substrate with                           suitable lattice match to MCT                           for epitaxial growth and is                           transparent to the range of                           IR wavelengths desired.    43   Heater            Provides controlled temperature                           gradient    44   Hotter portion of heater 43                           Interdiffuses more Cd per unit                           time into zone 40    45   Cooler portion of heater 43                           Interdiffuses less Cd per unit                           time into zone 40    ______________________________________

is a method of achieving a lateral variation in MCT film composition.The lateral variation in this embodiment may be quite long, such as 10cm, if desired, which is achieved by driving Cd into the MCT film fromthe substrate by interdiffusion. The desired x in a (Hg.sub.(1-x)Cd_(x))Te film 40 of (typically) uniform composition is grown on IRtransparent CdTe or CdZnTe substrate 42 as previously described. Thisensemble is placed on heater 43 which has a controlled temperaturegradient, for example, where portion 44 is hotter than portion 45. Thesetemperatures are selected for the desired properties of the modifiedfilm 40. The heat treated MCT film 40 in zone 39 has a larger x in the(Hg.sub.(1-x) Cd_(x))Te equation (more Cd) than does that of zone 41; asa result, the modified MCT film 40 will have a gradient of IR responsefor use in filters and detectors which varies from short wavelengths inzone 39 to longer wavelengths in zone 41 through the lateral dimensionof 40. Through the control of the temperature profile during theinterdiffusion heat treatment of 40, a physically long dimensioned IRfilter/detector may be achieved with any desired spectral responsewithin the IR spectrum.

Any MCT film having a compositional gradient in the film thicknessdirection may be converted to a compositional gradient in the lateraldirection by lapping the MCT film at a slight angle with respect to thefilm surface. FIG. 6A shows an exaggerated wedge of MCT film 40 whichhas been lapped on surface 46 to form a film which has varyingwavelength response in the lateral direction. For the purpose of examplethe MCT film 40 and substrate 42 are the same as described for FIG. 4. Amethod of delineating detectors having different spectral responses isshown in FIGS. 6B and 6C. In the example of FIG. 6B, a continuous MCTfilm 53 has been deposited over the lapped surface 46. Photoresist 47has been placed over film 53 and standard photolithography has beenperformed to produce detectors 48, 50 and 52, all of which havedifferent bandwidths and spectral centers because of the varyingcomposition through the thickness of MCT film 40.

Another method, shown in FIG. 6C, of creating these same detectors usesa mask to shield areas of MCT film 40 during the LPE growth process.First a continuous silicon dioxide (SiO₂) film 55 is deposited over MCTfilm 40. This oxide 55 is patterned into a mask by standard lithographyto etch holes where the detectors are to be located leaving SiO₂ islands54. Then, the ensemble is placed in an LPE reactor and the MCT materialfor detectors 48, 50 and 52 is deposited. The SiO₂ islands 54 preventsgrowth of MCT films by acting as an oxide mask.

After either of the detector delineation processes described for FIGS.6B or 6C, detector 48 will detect only the longest wavelengths, over therange of the width of detector 48; detector 50 will detect a wider IRband (because of its wider geometry) at the middle wavelengths; anddetector 52 will detect a narrow band of short IR wavelengths.

FIG. 7A is a sketch of a substrate holder 56 which holds an IRtransparent substrate 42 such that both sides of 42 are exposed to theMCT vapor in the LPE reactor shown in FIG. 2. The rectangular substrate42 and holder 56 shown is used as a typical example with theunderstanding that the opening to both 42 sides may be circular, squareor any other desired two dimensional geometry and that the holder may bedesigned for a plurality of substrates rather than just one. A crosssectional view of the substrate 42 after it has been exposed to the MCTvapor is shown in FIG. 7B for MCT films 64 and 70 on substrate 42.

As shown in FIG. 7B and Table 3, a very desirable, novel structureresults from

                  TABLE 3    ______________________________________    ID # Description      Function    ______________________________________    60   Radiant heater   Provides control over difference of                          annealing temperatures between                          substrate surfaces.    62   Hotter zone of MCT film 64                          Results in higher Cd concentration                          than zone 66 but less than 68.    64   MCT film of uniform                          Determines physical properties         composition      before heat treatment.    66   Cooler zone of MCT film 64                          Results in lower Cd concentration                          than zone 62 and 72.    68   Hotter zone within MCT film                          Results in the highest Cd         70               concentration of any zone in either                          64 or 70.    70   Same as MCT film 64 on the         other side of the substrate 42.    72   Cooler zone than 68 but hotter                          Results in Cd concentration lower         than zone 66.    than zone 68 but higher than 66.    ______________________________________

heat treatment of the two MCT film composite produced when using holder56. In this case substrate 42 with epitaxial films 64 and 70 having thesame MCT composition are placed on heater 43 having a hotter zone 44 andcooler zone 45 as described in FIG. 5. Heater 60 is optional and mayeither supply a constant temperature or graded temperature surface. InFIG. 7B, heater 60 is shown as a radiant heater and heater 43 is shownas a conduction heater but the choice is governed by the film propertiesdesired. In this example it is assumed that the average temperature ishotter within film 70 than within film 64. Therefore the Cdinterdiffusion from substrate 42 will produce a higher concentration ofCd within zone 68 than 62 directly above it. The lateral direction inboth MCT films 64 and 70 contain a gradient of decreasing Cd from zone62 to 66 and zone 68 to 72, respectively. The IR properties within zone68 attenuates or detects shorter wavelengths than the MCT material 62directly opposite it through IR transparent substrate 42. Through thevertical direction of the two films this results in a narrow band of IRspectrum characteristics with zone 68 attenuating the shorterwavelengths, but passing higher wavelengths to be detected/filtered byzone 62 at only a slightly higher wavelength than zone 68 blocks.Although described for the hotter temperature treated films in zones 62and 68, this same narrow spectral response at varying center wavelengthsexists across the entire lateral dimensions of the films.

The sketch of the device shown in FIG. 8 and described in Table 4 is anindication

                  TABLE 4    ______________________________________    ID#  Description    Function    ______________________________________    42   CdTe or CdZnTe Provide a substrate with suitable lattice         substrate      match to MCT for epitaxial growth and                        be transparent to the range                        of IR wavelengths desired.    64   Heat treated MCT film as                        Achieve IR response related to         described in FIG. 7B                        MCT film 70.    68   Highest Cd composition                        Attenuate lower and pass higher         of 70.         wavelengths of IR spectrum.    70   Heat treated MCT film as                        Achieve IR response related to         described in FIG. 7B                        MCT film 68.    72   Lowest Cd composition                        Attenuate longer IR wavelengths         of 70.         but pass those which will be                        detected immediately above by                        detector 90.    86   Detectors;     Patterned portions of film 64                        Form wideband IR detector for                        shorter IR wavelengths.    80,  Electrical contacts                        Forms electrical connection for external    92,  to detector 86 electronics to detect changes of    93                  conductance of 86 with                        IR radiation.    88   Patterned portion of                        Form medium bandwidth IR detector at         film 64        medium wavelengths of IR spectrum.    82, 9         Electrical contacts                        Forms electrical connection for external    4, 95         to detector 88 electronics to detect changes of                        conductance of 88 with                        IR radiation.    90   Patterned portion                        Form very narrow bandwidth, long         of film 64     wavelength IR detector.    84, 9         Electrical contacts                        Forms electrical connection for external    6, 97         to detector 90 electronics to detect changes of                        conductance of 90 with                        IR radiation.    98   Incident IR radiation    ______________________________________

of the wide utility that may be obtained from the fabrication methodsshown in FIGS. 7A and B.

MCT films 64 and 70 are LPE deposited and heat treated as described inFIG. 7B such that a point lying on MCT film 64 has slightly less Cd thanthe same lateral location on 70. Film 64 is patterned into detectors 86,88 and 90 (FIG. 8) with attached electrical conductors 80,92 and 82,94and 84,96 respectively. An increase of conductance in these detectorsoccurs when irradiated with the proper wavelengths of IR. The lateralpositions of detectors 86, 88 and 90 determine the center of their IRspectral response. The physical widths of detectors 86, 88 and 90determine their bandwidths. For example, the width of detector 86 maydetect wavelengths having lower limits from 3 mm on the left to 4.5 mmon the right and the composition of film 70 immediately opposite todetect 86 may pass wavelengths longer than 2.8 mm on the left and 4.3 mmon the right. Thus, detector 86 may exhibit increases in conductance foronly the band of 3 to 4.5 mm. Detector 88 has a similarly tailoredresponse at longer wavelengths with a narrower passband than detector86. Detector 90 has a very narrow range of IR wavelength response at alonger wavelength than either detectors 86 or 88 which is desirable whendetecting carbon dioxide or carbon monoxide components during combustionof organic material in an atmospheric environment.

An advantage of the present invention, a double sided filter/detectordevice is that a substantial inventory of double sided film material maybe prepared in advance. Subsequently the MCT film 64 may be patterned toachieve an IR response tailored to the application.

For system design considerations where the cost permits, it may bedesirable to have the MCT filters and detectors on the same substratewith the active devices that process the signals from the detectors. Thecrystal lattice mismatch between Si and MCT is much too severe to permiteither to be grown on the other. (e.g. Si grown on MCT or vice versa)However, FIG. 9 and Table 5 illustrate two methods of manufacturingactive devices and MCT filters and detectors on the same substrate.Although only one MCT film is shown, it should be understood that aplurality of MCT films acting as either or both filters and detectors ispossible because the MCT growth temperature of around 450° C. will notharm either unbonded GaAs or unbonded Si devices. In FIG. 9A anepitaxially grown film of GaAs 106 is grown on Si substrate 110. Whereit is desired to form Si devices in region 112, the GaAs is etched away.Region 112 is fabricated with either or both discrete and integrated Sidevices and then covered with a protective layer, such as plasmadeposited silicon nitride (not shown) while GaAs devices are created inthe GaAs region 108. Both 106 and 110 are covered

                  TABLE 5    ______________________________________    ID#  Description       Function    ______________________________________    100  MCT film layer    To be patterned into IR detectors    102  CdTe film layer   Lattice match between 100 and                           106 permits LPE film growth.    104  Plasma deposited silicon nitride                           Prevents damage to regions 108         or silicon oxide protective layer                           and 112 which contain active                           semiconductor devices.    106  GaAs epitaxial layer                           Can be epitaxially grown on Si                           and can also provide useful                           devices within 108.    108  Protected GaAs device region                           May contain discrete or inte-                           grated circuit devices of GaAs.    110  Silicon substrate Provides mechanical support for                           all epitaxially grown layers                           and may provide region 112.    112  Protected Si device region                           May contain discrete or inte-                           grated circuit devices of Si.    114  ZnSe epitaxial layer                           Provides lattice parameters                           which allows it to be epitaxially                           grown on Si and also allows                           CdTe to be epitaxially                           grown on it.    ______________________________________

with a protective layer 104, such as plasma deposited silicon nitride,during the subsequent LPE growth phase of 102 and 100. CdTe 102 may begrown epitaxially on GaAs 106 and MCT 100 may be grown on 102. Althoughthe variants are numerous, MCT 100 may now be fabricated into detectorshaving the desired IR wavelength response without harm to the protectedcircuits 108 and 112. The protective layer 104 may now be selectivelyremoved and interconnecting bonding pads opened such that leads may beattached or deposited between 100, 108 and 112 for the final systemconfiguration.

FIG. 9B shows a similar configuration where only Si devices arerequired. In this case the Si substrate 110 has silicon devicesfabricated in region 112 as before and then covered overall with aprotective layer of plasma deposited silicon nitride 104. The exposedarea of Si 110 matches the lattice parameters of ZnSe close enough toallow the epitaxial growth of ZnSe film 114. ZnSe also matches thelattice parameters of CdTe close enough to allow the epitaxial growth ofCdTe film 102. Now the MCT detector/filter film 100 is grown on 102 andpatterned into the desired detector/filter geometries. Layer 104 may beselectively removed to expose the bonding pads of 112 and the such thatinterconnection leads may be bonded between the Si devices of 112 andthe detectors formed from 100.

FIG. 9c shows an integrated device with HgCdTe filter, HgCdTe IRsensors, and silicon circuitry, which can be used, e.g., as a integratedspectrometer. Layer 104 has been selectively removed to form a via toexpose the bonding pads of 112. The MCT film 100 (see FIG. 9b) has beenpatterned (e.g. photolithographically etched) to form MCT detectorstripes 101, 121. Via conductor 116 has been formed (e.g. from a metalsuch as aluminum) in the via and interconnection lead 118 has been usedto connect sensor 121 to via conductor 116, thus connecting the Sidevices of 112 and the detectors formed from 100. Interconnection lead118 may be from a metal such as aluminum, but is preferably HgTe, andlead 118 is patterned to cover only a small end portion of sensor 121and to leave most of the sensor uncovered. A second lead (not shown) canbe used to make a second contact at the opposite end of sensor 121. CdTeinsulator 120 has been LPE grown between and over the stripes and MCTfilter film 122 LPE grown over the CdTe insulator.

While the filter film could be continuously graded at this point withcontrolled temperature gradient heater 43 of FIG. 5, FIG. 9d shows analternate embodiment in which non-uniform MCT filter film 122 isfabricated using a mask 124 and conventional rapid thermal annealinglamps 126. This embodiment can be used in controlled atmospherecontaining Hg vapor. The mask contains large openings 128 to providehotter portions of MCT filter film 122, small openings 130, to provideintermediate temperature portions of MCT filter film 122, and solidportions 132 to provide cooler portions of MCT filter film 122.Preferably a non-contact mask is utilized, and the lamp-mask spacing isnot to scale, as the lamps are generally much further away than shown.While rapid thermal annealing lamps have previously been used to provideuniform heating, here the lamps are used in conjunction with areflective mask to provide non-uniform heating. Note that while the maskis shown with holes, it could also be a patterned reflective coating ona transparent substrate. Note also such lamps can also be used inconjunction with a mask to provide nonuniform heating to MCT film 100(see FIG. 9b) prior to the application of CdTe insulator 120, eitherbefore or after film 100 has been patterned to form MCT detector stripes(stripes 101, 121 as shown in FIG. 9c). Such nonuniform heating of film100 or stripes 101, 121 can provide for MCT detectors having differentspectral responses.

FIG. 9e shows and alternate embodiment in which a controlled atmospherecontaining Hg vapor is not used. Instead, the structure of FIG. 9c hasbeen modified by adding a HgTe layer 132 over the MCT filter film 122.Again, non-uniform MCT filter film 122 is fabricated using mask 124 andconventional rapid thermal annealing lamps 126. Generally the HgTe layeris removed after the heating. This embodiment can be used to provide anintegrated IR detector assembly with a continuously graded HgCdTe filterlayer without the use of a controlled atmosphere containing Hg vapor.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. For illustrative purposes, specifically tuned sensorsand geometric arrangements have been chosen for clarity without in anyway intending that these examples limit the generic application of MCTIR detectors operated at either uncooled or cooled temperatures todetect IR for analytic or control purposes. As used herein, the term"uncooled" includes ambient temperature usage as well as temperaturescontrolled within 20 degrees C. of ambient. The example applications areintended as prototypical and not limited to those included. Many moredetectors than shown can be placed on the same substrate. An analyticinstrument such as an IR spectroscope would contain numerous MCTdetectors of variously tuned wavelengths. Non-uniform heating coulde.g., also be provided by a scanning laser. Various modifications andcombinations of the illustrative embodiments, as well as otherembodiments of the invention, will be apparent to persons skilled in theart upon reference to the description. It is therefore intended that theappended claims encompass any such modifications or embodiments.

We claim:
 1. A method of fabricating an integrated infrared detectorassembly, comprising the steps of:epitaxially growing a verticallycontinuously graded HgCdTe layer, and thereafter physically removingmaterial from the HgCdTe layer so that the HgCdTe layer is laterallycontinuously graded.
 2. The method of claim 1, wherein the HgCdTe layeris a filter layer, and including the steps of:epitaxially growing anHgCdTe detector film and an electrically insulating layer, theelectrically insulating layer being disposed between the HgCdTe layerand the HgCdTe detector film; and removing material from the HgCdTedetector film to form at least two HgCdTe detectors.
 3. The methodaccording to claim 2, wherein said step of removing material from theHgCdTe detector film is carried out by etching the HgCdTe detector film.4. The method of claim 1, wherein said step of physically removingmaterial from the vertically continuously graded HgCdTe layer includesthe step of diamond point turning the vertically continuously gradedHgCdTe layer.
 5. A method of fabricating an integrated IR detectorassembly, said method comprising:liquid epitaxially growing in apredetermined sequence a vertically continuously graded HgCdTe filterlayer, an electrically insulating layer, and a HgCdTe detector film; andetching said HgCdTe detector film to form at least two HgCdTe detectors,and diamond point turning said vertically continuously graded HgCdTefilter layer to form a laterally continuously graded HgCdTe filterlayer.
 6. The method of claim 5, wherein said electrically insulatinglayer is CdTe or CdZnTe.
 7. The method of claim 5, wherein saiddetectors have end-portions and said end-portions have HgTe contactsthereon.
 8. The method of claim 5, wherein said laterally continuouslygraded HgCdTe filter layer is on an IR transmissive window.
 9. Themethod of claim 8, wherein said IR transmissive window is CdTe.
 10. Themethod of claim 5, wherein in said liquid epitaxially growing, saidvertically continuously graded HgCdTe filter layer is grown first, saidelectrically insulating layer is grown next, and said HgCdTe detectorfilm is grown last.
 11. The method of claim 5, wherein in said liquidepitaxially growing, said HgCdTe detector film is grown first, saidelectrically insulating layer is grown next, and said verticallycontinuously graded HgCdTe filter layer is grown last.
 12. The method ofclaim 5, wherein in said etching of said HgCdTe detector film to form atleast two HgCdTe detectors is performed prior to said diamond pointturning of said vertically continuously graded HgCdTe filter layer. 13.An integrated IR detector assembly, said assembly comprising:a laterallycontinuously graded HgCdTe filter layer; an electrically insulatinglayer provided directly on said continuously graded HgCdTe filter layer;and at least two integral HgCdTe detectors provided directly on saidelectrically insulating layer; wherein said continuously graded HgCdTefilter layer is wedge-shaped and is also continuously graded vertically.14. The assembly of claim 13, wherein said detectors have end-portionsand said end-portions have HgTe contacts thereon.
 15. The assembly ofclaim 13, wherein said laterally continuously graded HgCdTe filter layeris on an IR transmissive window.
 16. The assembly of claim 15, whereinsaid IR transmissive window is CdTe.
 17. An integrated IR detectorassembly, said assembly comprising:a laterally continuously gradedHgCdTe filter layer; an electrically insulating layer provided directlyon said continuously graded HgCdTe filter layer; and at least twointegral HgCdTe detectors provided directly on said electricallyinsulating layer; wherein said electrically insulating layer is CdTe orCdZnTe; and wherein said continuously graded HgCdTe filter layer iswedge-shaped and is also continuously graded vertically.